From the passage we understand that Carl Sagan writes （）

A．is very confused about the origin of the Earth
B．might possibly reach life outside the
C．believes it is possible to step off the Earth
D．is historically incapable of exploring the universe

A．students should learn more up-to-date knowledge
B．lack of scientific knowledge will lead to poor relations between producers and users of scientific services
C．students are getting to know the importance of a good balance among the branches of knowledge
D．sufficient attention should be given to bask knowledge